This invention relates to a fuel processor for generating hydrogen gas by reforming hydrocarbon-based fuels, and more particularly, to a method and apparatus for heating fuel processor components, such as a water-gas-shift reactor, during startup of the fuel processor.
A fuel cell is a device that converts chemical energy directly into electrical energy and heat. In perhaps its simplest form, a fuel cell comprises two electrodesxe2x80x94an anode and a cathodexe2x80x94separated by an electrolyte. During use, a fluid distribution system supplies the anode with fuel and supplies the cathode with an oxidizer, which is usually oxygen in ambient air. With the aid of a catalyst, the fuel undergoes oxidation at the anode, producing protons and electrons. The protons diffuse through the electrolyte to the cathode where, in the presence of a second catalyst, they combine with oxygen and electrons to produce water and heat. Because the electrolyte acts as a barrier to electron flow, the electrons travel from the anode to the cathode via an external circuit containing an electrical load that consumes power generated by the fuel cell. A fuel cell generates an electrical potential of about one volt or less, so individual fuel cells are xe2x80x9cstackedxe2x80x9d in series to achieve a requisite voltage.
Because fuel cells are more efficient than heat engines and can generate electricity with zero or near zero emission of pollutants, researchers have proposed replacing internal combustion engines in vehicles with fuel cells. Among the fuels that have been considered for vehicle applications, hydrogen (H2) appears to be the most attractive. Hydrogen has excellent electrochemical reactivity, provides sufficient power density levels in an air-oxidized system, and produces only water upon oxidation. Despite these advantages, however, its use in vehicles is hampered by on-board storage difficulties and by the lack of an established retail supply network of H2.
For these reasons, fuel cell engine designs often include a fuel processor, which employs steam reforming, autothermal reforming or partial oxidation to convert conventional hydrocarbon-based fuels, such as gasoline and methanol, to hydrogen. Most fuel processors include a primary reactor, a water-gas-shift (WGS) reactor, and a preferential oxidation (PrOx) reactor to generate xe2x80x9cstack gradexe2x80x9d H2. In steam reforming the fuel processor supplies the primary reactor with water (steam) and a hydrocarbon-based fuel (e.g., gasoline, methanol, etc.), which react to form a mixture of H2, carbon dioxide (CO2), carbon monoxide (CO), and excess steam. Since CO would poison the anode catalyst, the fuel processor channels the primary reactor effluent (reformate) to the water-gas-shift (WGS) reactor, which contacts the gas mixture with a catalyst and water to convert most of the CO to CO2 and H2. Finally, the fuel processor converts residual CO to CO2 in the PrOx reactor, which comprises a catalyst bed operated at temperatures (e.g., 150xc2x0 C. to 250xc2x0 C.) that promote preferential oxidation of CO by air with little attendant oxidation of H2. In steam reforming, fuel gas leaving the PrOx reactor typically contains (in mole %) about 70% H2, 24% CO2, 6% nitrogen (N2) and trace amounts ( less than 20 ppm) of CO.
Autothermal reforming and partial oxidation share many features of steam reforming. For example, in one form of autothermal reforming, a portion of the hydrocarbon-based fuel may be burned or partially oxidized with oxygen or air within a reaction zone that is physically separate from the reforming reaction. Heat from the oxidation drives the endothermic conversion of water and the balance of the hydrocarbon-based fuel to H2, CO2, and CO in the reforming reaction zone. In another form of autothermal reforming, a portion of the hydrocarbon-based fuel is oxidized in the same reaction zone as the reforming reaction. Similarly, in partial oxidation, a fuel-rich mixture of the hydrocarbon-based fuel and air are reacted in the primary reactor, producing a gas mixture comprised mainly of H2, CO2, and CO. Autothermal reforming and partial oxidation also utilize WGS and PrOx reactors to reduce CO levels in the reformate stream leaving the primary reactor; the final reformate composition is about 42% N2, 38% H2, 18% CO2, less than 2% methane (CH4), and less than about 20 ppm CO. For further details of fuel processors for generating stack-grade H2, see U.S. Pat. No. 6,077,620 entitled xe2x80x9cFuel Cell System with Combustor-Heated Reformer,xe2x80x9d which is herein incorporated by reference in its entirety and for all purposes.
One challenge facing developers of fuel cell engines is the ability to rapidly generate stack grade H2 upon starting the fuel processor at ambient temperature (cold start conditions). Though many factors may affect fuel processor startup, it is particularly limited by the time required for the reactors to reach their operating temperatures. For example, a low temperature water-gas-shift reactor must reach about 200xc2x0 C. before it can reduce CO in the reformate stream to levels low enough to be tolerated by the PrOx reactor and the fuel cell stack. A high temperature water-gas-shift catalyst must be even hotter (about 350xc2x0 C.). Typically, the only heat available for raising the temperature of the water-gas-shift reactor is the sensible heat of the primary reactor effluent. This heat must be used to raise the temperature of the entire thermal mass downstream of the primary reactor, including the WGS reactor, the PrOx reactor and any heat exchangers.
Fuel processor startup is complicated by the presence of water vapor in the primary reactor effluent and the WGS reactor feed stream. Since water vapor may condense on the cold WGS catalyst, additional energy must be supplied during startup to vaporize any condensed water before the WGS catalyst is heated. Although the fuel processor may be run without water injection during startup to limit water vapor condensation, such practice may result in the primary reactor reaching excessive temperatures. As noted above, even if there is no water in the fuel processor feed at startup, the primary reactor generates water that may condense in the water-gas-shift reactor. Similarly, water may also condense on the cold PrOx catalyst during cold start, thus requiring additional energy to revaporize the condensed water.
Researchers have proposed several techniques for increasing heating rates of the fuel processor reactors, but each method has drawbacks. For example, the water-gas-shift reactor may be electrically heated at startup, but electric heating requires a secondary power supply that adds to the cost of the fuel processor. Alternatively, air or oxygen may be injected into the primary reactor effluent as it enters the water-gas-shift reactor, and an electrically heated catalyst (EHC) may be used to combust the H2 and CO in the primary reactor effluent and subsequently heat the water-gas-shift catalyst. However, an EHC requires a secondary power supply, and air or oxygen injection may result in a loss of catalyst activity since many WGS catalysts are sensitive to oxygen. Non-pyrophoric water-gas-shift catalysts that xe2x80x9clight offxe2x80x9d or react in the presence of oxygen can generate sufficient heat to start the water-gas-shift reaction. However, such catalysts contain costly precious metals and still need to reach a light-off temperature of about 130xc2x0 C. to become active.
The present invention overcomes, or at least mitigates, one or more of the problems discussed above.
The present invention provides an apparatus and method for supplying additional heat to fuel processor componentsxe2x80x94including the water-gas-shift reactorxe2x80x94during startup of the fuel processor at ambient temperatures. The additional heat is supplied without expending secondary power, and is accompanied by the removal of water from the fuel processor""s primary reactor effluent. The added heat allows the water-gas-shift reactor to reach its operating temperature more rapidly, which reduces the time needed for the fuel processor to generate stack grade H2 during startup. In addition, by removing water from the primary reactor effluent, condensation of water vapor on the water-gas-shift reactor catalyst during startup is reduced or eliminated, which obviates the need to supply additional heat to vaporize the condensed water.
Therefore, one aspect of the present invention provides a fuel processor comprised of a primary reactor and a water-gas-shift reactor. The primary reactor is adapted to convert a hydrocarbon-based fuel to hydrogen, carbon dioxide, carbon monoxide and water. The water-gas-shift reactor contains a catalyst that is adapted to convert at least a portion of the carbon monoxide in the primary reactor effluent to carbon dioxide and hydrogen. The inlet to the water-gas-shift reactor communicates with the outlet of the primary reactor. The fuel processor also includes a water adsorbent that is located within a flow path between the outlet of the primary reactor and the outlet of the water-gas-shift reactor. During fuel processor startup, the water adsorbent generates heat by adsorbing at least a portion of the water in the primary reactor effluent. Useful water adsorbents include zeolites.
Another aspect of the present invention provides a method of heating a fuel processor during startup. The fuel processor includes a primary reactor that converts a hydrocarbon-based fuel to H2, CO2, CO, and H2O, and a water-gas-shift reactor, which in the presence of a catalyst, converts at least some of the CO and H2O from the primary reactor to CO2 and H2. The method includes providing a water adsorbent within a flow path between an outlet of the primary reactor and an outlet of the water-gas-shift reactor. The water adsorbent generates heat during startup of the fuel processor by adsorbing at least a portion of the H2O from the primary reactor. Preferably, the heat generated is sufficient to raise the temperature of the water-gas-shift catalyst to a point where the catalyst can be lit-off by injecting oxygen or air into the water-gas-shift reactor. To maintain adsorption capacity following fuel processor shutdown, the method may also include purging water from a void volume adjacent the water adsorbent using a dry gas, e.g. from the interstices between palletized adsorbents, or from the cells of a monolithic adsorbent.